1. Field of the Invention
The invention relates to melting metal particles and, more particularly, to a method and apparatus for rapidly melting scrap metal particles of light metals such as aluminum within a crucible.
2. Description of the Prior Art
Light gauge, low density scrap metal particles such as chips, borings, and turnings are produced as a by-product of many metal processing operations. A significant amount of scrap metal also exists in the form of metal cans, particularly aluminum cans. For convenience, all such scrap metal will be referred to herein as "scrap metal" and "particles." In order to recover the scrap metal for productive use, it is necessary to remelt it. Unfortunately, a number of problems are presented when scrap metal is attempted to be remelted. These problems are particularly acute in the case of light metals such as aluminum due to the tendency of the metal to oxidize when melted. The problems are worse for small particles of scrap metal than large ones, because (1) small particles have a relatively large surface-to-volume ratio and (2) small particles tend to remain on the surface of a melting bath where they are oxidized while large particles sink rapidly beneath the surface without oxidizing.
Reverberatory furnaces have been used to melt scrap metal, but mechanical puddlers are necessary to achieve respectable recovery rates when small particles of scrap metal are being melted. Puddlers are expensive, bulky, mechanically complex, and are a source of iron contamination. Even with mechanical puddlers, melting of the scrap metal occurs slowly so that the metal tends to oxidize before it melts, resulting in recovery rates that are less than desirable. "Recovery rate" as used herein can be defined as follows: ##EQU1##
The situation is improved when induction furnaces are used. Strong inductive currents are set up in the molten metal which create a stirring action that rapidly submerges the scrap metal before additional oxide can form on the surface. Furthermore, the absence of high temperature combustion produces little or no oxide formation. The result is that recovery rates on the order of 97 percent can be attained. The chief drawback of the induction furnace melting technique is the high initial cost of the furnace. The cost can be so great as to make the scrap recovery process uneconomical despite the high recovery rates available. A further drawback of the induction furnace melting technique is that it is a batch process, rather than a continuous process.
A different approach to the problem of recovering scrap metal is disclosed in U.S. Pat. No. 3,272,619 (hereafter the '619 patent), to V. D. Sweeney et al, the disclosure of which is incorporated herein by reference. In the '619 patent, molten metal is circulated from a reverberatory furnace, through an external crucible where a vortex is established, and back into the furnace. Melting of scrap metal does not occur in the furnace. Rather, the scrap metal is introduced into the vortex established in the external crucible. As the scrap metal swirls down in the vortex, the scrap metal particles eventually are melted. By appropriate control of such parameters as the temperature of the molten metal being circulated, the moisture content of the particles, and the rate at which the particles are fed into the crucible, recovery rates of about 90 percent can be attained.
Although the system described in the '619 patent has been reasonably effective, certain problems have not been addressed. The '619 patent states that the intensity of the vortex can be adjusted to produce desired submerging rates, but such adjustment has proven difficult to achieve in practice. The high surface tension of the molten metal in the crucible permits solid particles to remain on the surface of the vortex completely down into the return pipe to the furnace. The result is that solids and air can reach the furnace, with a consequent lowering of melting efficiency. In effect, the scrap metal being melted is exposed excessively to air such that undesired quantities of dross are formed. It is possible that oxide-covered metal drops (referred to hereafter as "agglomerations") can pass completely through the crucible and back into the furnace. An additional concern related to the device according to the '619 patent is the sensitivity of the crucible to flow variations. Because the crucible is most efficient with metal flowing near the top, a slight increase in flow rate can cause a spillover. Additionally, such a high operating level in the crucible can cause loss of heat through the crucible itself.